Endocrine

From Iusmhistology

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Revision as of 00:55, 7 April 2011

• started here on 03/24/11.

Contents 1 Endocrine histology 1.1 Describe the structural organization of the endocrine system 1.2 Define components of the endocrine system 1.3 Describe the embryonic origin, histological organization, and hormone secretion of the endocrine system 1.3.1 Origin, organization, and secretion of the hypothalamus 1.3.2 Origin, organization, and secretion of the pituitary gland (hypophysis) 1.3.2.1 Adenohypophysis (anterior pituitary) 1.3.2.2 Neurohypophysis (posterior pituitary) 1.3.2.3 Pituitary portal system 1.3.2.4 Clinical correlate: growth hormone deficiency 1.3.3 Origin, organization, and secretion of the Adrenal glands

Endocrine histology

Describe the structural organization of the endocrine system

• The endocrine has several specific characteristics:
  • cells of epithelial origin secrete hormones onto endothelial tracts (that is, the blood stream)
  • hormones act at distance sites defined by having the receptor for the hromone
  • secretory cells are localized to endocrine organs
  • hormones can be classified into one of several grooups: amino acids, peptides, steroids, or proteins / glycoproteins
• Note that exocrine glands secrete onto an epithelial surface that is usually in the form of a duct whereas endocrine glands secrete into the blood stream.
  • Furthermore, both exocrine glands and endocrine glands are usually on the outside of the basement membrane relative to the blood.
  • Therefore, exocrine glands do not secrete across the basement membrane.
  • However, endocrine glands often must secrete their hormones across the basement membrane.

Define components of the endocrine system

• There are three types of endocrine components: full endocrine organs, endocrine components as part of other solid organs, and diffuse endocrine components.
• Full endocrine organs are those organ in which the primary function is to synthesize, store, and secrete hormones.
  • Think pituitary gland.
• Endocrine components as part of a solid organ are those organs in which there are clusters of endocrine cells.
  • Think pancreas.
• Diffuse endocrine components occur when individual endocrine cells are scattered (or clumped) within an extensive epithelium.
  • Think adipose tissue and sabaceous glands.

Describe the embryonic origin, histological organization, and hormone secretion of the endocrine system

Origin, organization, and secretion of the hypothalamus

• The hypothalamus resides in the lower, central part of the brain and houses neurosecretory neurons that produce hormones.
• Neurosecretory neurons secrete hormones into the hypothalamic-hypophysis portal system, a double capillary bed that facilitates potent delivery of hormones from the hypothalamus to the hypophysis (pituitary).
• The hypothalamus releases 5 hormones from three nuclei (dorsal medial, ventral medial, and infundibular nuclei):
  • GnRH (gonadotropin releasing hormone) which stimulates gonadotropes of the anterior pituitary to release LH and FSH.
  • TRH (thryroid releasing hormone) which stimulates thyrotropes and mammotropes (lactotropes) of the anterior pituitary to release TSH and PRL.
  • PIF (prolactin inhibitory factor, dopamine) which inhibits lactotropes (mammotropic cells) of the anterior pituitary from releasing PRL.
  • CRH (corticotropin releasing hormone) which stimulates corticotropes of the anterior pituitary (adenohypophysis) to transcribe POMC and release ACTH and beta-LPH.
  • GHRH (growth hormone releasing hormone) which stimulates somatotropes of the anterior pituitary.
  • SST (somatostatin) which inhibits somatotropes and thyrotopes of the anterior pituitary (adenohypophysis) to release GH and TSH.
• The hypothalamus also contains two more nuclei that produce two other hormones that are delivered directly to the posterior pituitary (neurohypophysis) through the axons of the neuron cells that produce the hormones.
  • The supraoptic neucleus produces vasopressin (ADH, AVP) which acts on the collecting ducts of the kidney (think AQ2).
  • The paraventricular nucleus produces oxytocin which acts on the mammary glands (myoepithelial cells) and uterus (smooth muscle cells, contractions).
    • Oxytocin may be associated with increased generocity.

Origin, organization, and secretion of the pituitary gland (hypophysis)

• The hypophysis (pituitary gland) is located below the brain in the sella turcica, a cavity of the sphenoid bone.
• The hypophysis has two regions: the adenohypophysis (anterior pituitary) and the neurohypophysis (posterior pituitary).
  • The adenohypophysis originates from the oral ectoderm.
    • This makes sense because it has "ad" "deno" which means "toward the teeth".
  • The neurohypophysis originates from the brain.
    • This makes sense when one recalls that it is the posterior pituitary (neurohypophysis) into which neurons from the hypothalamus directly connect and directly release hormones.
    • Also, knowing that the neurohypophysis (posterior pituitary) originates from brain tissue makes sense because one of the hormones released by the posterior pituitary (neurohypophysis) has a very neurotransmitter-like role: oxytocin causes muscle contraction.

• The pituitary gland's two divisions have several components:
  • The anterior pituitary consists of the pars distalis and the pars tuberalis.
  • The pars intermedia separates the two functional units.
  • The posterior pituitary (neurohypophysis) is made up of the pars nervosa and the infundibular stalk.
• The infundibulum is the combination of the infudibular stalk and the pars tuberalis, from the neurohypophysis and adenohypophysis, respectively.

• Embryology of the pituitary gland
  • Recall that the adenohypophysis arises from the oral ectoderm.
    • The oral ectoderm (roof of the mouth) grows caudally, forms Rathke's pouch, and eventually separates the pouch from the oral ectoderm.
    • Recall that this caudal pouching of the oral ectoderm generates the pars tuberalis, the pars distalis, and the pars intermedia.
  • Recall that the neurohypophysis arises form the brain tissue.
    • The neuroectoderm (floor of the diencephalon) grows caudally, forms a stalk, and remains attached to the brain tissue of origin.
    • Recall that this caudal stalk formation generates the pars nervosa, the median eminance, and the median eminence.

Adenohypophysis (anterior pituitary)

• The adenohyophysis is composed of the pars distalis, the pars tuberalis and the pars intermedia.
  • The pars distalis is the same as the anterior lobe.

• The pars distalis (anterior lobe):
  • The pars distalis is composed of fibroblast generated reticular fibers that support hormone-generating epithelial cells and a rich bed of fenestrated capillaries.
  • Cells of the pars distalis can be classified by the way the stain: basophilic, acidophilic, and chromophobes.
  • Acidophilic cells: somatotropes and mammotropes (lactotropes).
  • Basophilic cells: gonadotropes, croticotropes, and thyrotropes
  • Chromophobic cells: stem cells, degranulated cells that would otherwise be chromophilic (see acidophilic and basophilic).
  • Differentiating cell types is not possible with light microscope, only by trasmission electron microscopy can these hormone producing cells be differentiated.

• The pars tuberalis:
  • This is a funnel shaped structure that surrounds the infudibular stalk of the neurohypophysis.
  • Most cells of the pars tuberalis are basophilic.

• The pars intermeida:
  • The pars intermedia the lumenal remnant of the pouch part of Rathke's pouch.
  • The pars intermeida separates the pars distalis of the adenohypophysis and the pars nervosa of the neurohypophysis.
  • Colloid-filled cysts fill the pars intermedia.

Neurohypophysis (posterior pituitary)

• The neurohypophysis is derived from the neuroectoderm and contains two major regions: the pars nervosa and the infundibular stalk.
• The neurohypophysis contains nerve cells and glial cells (pituicytes).

• The pars nervosa:
  • The pars nervosa contains fibroblasts, pituicytes, mast cells and neurons.
  • The neurons arise from the paraventricular and supraoptic neuclei where oxytocin and vasopressin are made, respectively.
  • These neurons are atypical in that they do not synapse at their distal axons.
  • The hormones released by these neurons are stored in granules (called Herring bodies or neurosecretory bodies) at the distal aspect of the axon.
    • Herring bodies can be identified under light microscopy.

• The infundibular stalk
  • The infundibular stalk, like the pars nervosa, contains atypical nerve axon endings that release hormones.
  • The neurons of the infundibular stalk release their hormones into the hypothalamus-pituitary portal system and affect the cells of the anterior pituitary.

Pituitary portal system

• There are superior, middle, and inferior hypophyseal arteries that service the adenohypophysis.
• There are really 4 main components to the portal system: primary and secondary capillary beds, long veins and short veins.
• The primary capillary bed arises from the superior hypophyseal artery and resides around the median eminance.
  • At the primary capillary bed, neurons of the hypothalamus dump hormones into the blood stream.
• The long veins connect the primary capillary bed to the secondary capillary bed.
• The secondary capillary bed resides around the adenohypophysis.
  • At the secondary capillary bed, hormones from the hypothalamus exit to affect the cells of the anterior pituitary and hormones form the anterior pituitary (adenohypophysis) enter the blood stream.
• The inferior hypophyseal artery forms a capillary mesh at the neurohypophysis.
• The short veins connect the capillaries of the neurohypophysis to the secondary capillary bed of the adenohypophysis.
  • It is unclear if there is a particular function associated with this connection between the neurohypophysis and adenohypophysis.

Clinical correlate: growth hormone deficiency

• Growth hormone deficiency results in low levels of GH release and therefore low levels of IGF1, IGF2, and IGF-binding protein 3 from the liver.
• Decreased levels of IGFs results in decreased growth and stature and delayed physical maturation.

Origin, organization, and secretion of the Adrenal glands